The present invention relates to a medium-transporting apparatus.
With conventional tandem type image-forming apparatuses such as a color electrophotographic printer, a print medium passes through image-forming sections for corresponding colors. A print medium is advanced by a medium pick up roller from a print medium cassette, and fed by feed rollers to transport rollers. The transport rollers transport the print medium to a carrier belt and the print medium is attracted by the Coulomb force to the carrier belt. The image-forming sections for yellow, magenta, cyan, and black images face the carrier and are disposed along the carrier belt. Toner images of corresponding colors are transferred in registration with one another from the photoconductive drums of the respective image-forming sections. The print medium is then separated from the carrier belt and directed to a fixing unit where the colored toner images are fixed into a full color image.
In order for the carrier belt to attract the print medium, a voltage is applied across an attraction roller and an idle roller that opposes the attraction roller. When the print medium passes an attraction region between the attraction roller and the carrier belt, the print medium on the carrier belt is charged. A current flows through the print medium that functions as a capacitance component. This capacitance component stores the Coulomb force that attracts the print medium to the carrier belt.
The attraction roller longitudinally extends over a length longer than the width of the print medium.
When the width of a print medium is shorter than the length of the attraction roller, the attraction region has an area in direct contact with the carrier belt and areas in which the print medium is sandwiched between the attraction roller and the carrier belt. If the print medium has a higher resistance than the attraction roller, a larger current flows through the areas of the attraction roller in direct contact with the carrier belt than through the areas in which the print medium is sandwiched between the carrier belt to the attraction roller.
Therefore, a sufficient voltage cannot be applied across the thickness of the print medium with the result that the print medium is not sufficiently attracted to the carrier belt. To overcome this drawback, the electrical resistance of the attraction roller can be made higher than that of the print medium.
However, increasing the electrical resistance of the attraction roller results in larger voltage drops across the attraction roller. The large voltage drops across the attraction roller necessitates a higher output voltage of a power supply to ensure that a sufficiently high voltage is applied across the thickness of the print medium.